CommandSource Additions & Potion API

Added command sources to the API regarding this issue: http://issues.spongepowered.org/youtrack/issue/SPONGE-45

I can not come up with any command methods that will be shared so no methods were added to the CommandSource interface. I did, however, create 5 CommandSource representing different sources. Some of the sources have a basic implementation added.

Additions:

CommandBlockSource: Command comes from command block.
ConsoleSource: Command comes from server console.
PlayerSource: Command comes from a player (some methods implemented).
RconSource: Command comes from RCON.
WebSource; Command comes from web client.

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Added a basic Potion API. More methods will be added relating to applying potions once there is an entity implementation.

Additions:

PotionEffect: The effects that potions will apply to entities
PotionType: A type of potion which contains PotionEffect's 
Potion: Takes use of a PotionType and applies it to an entity.